The place where Alexandria was going to meet with Rein was inside a large fortress-like structure and it was not inside the dungeon anymore. After Lucia took over the Deity's Resting Ground dungeon, all the trapped gods were freed from it. Most of them decided to join Alexandria in her undead legions and only a few decided to accept the destruction of their souls. Rein fulfilled their wishes.
Now the Deity's Resting Ground was a brand new dungeon under a brand new leader, but the adventurers who regularly entered the dungeon had no idea about it. Lucia also did not bother to make any drastic changes so no one even noticed that something had changed with the dungeon.